We discuss the formation and evolution of systems composed of a low-mass (M ⋆ ≲ 4 M ⊙) main-sequence star orbiting a 105-107 M ⊙ supermassive black hole with an orbital period of order ∼hours and a mild eccentricity (e ≈ 0.1-0.2), episodically shedding mass at each pericenter passage. We argue that the resulting mass transfer is likely unstable, with Roche lobe overflow initially driven by gravitational-wave emission, but then being accelerated by the star’s expansion in response to its mass loss, undergoing a runaway process. We show that such systems are naturally produced by two-body gravitational encounters within the inner parsec of a galaxy, followed by gravitational-wave circularization and inspiral from initially highly eccentric orbits. We argue that such systems can produce recurring flares similar to the recently identified class of X-ray transients known as quasiperiodic eruptions, observed at the centers of a few distant galaxies.
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© 2023. The Author(s). Published by the American Astronomical Society.